2 Cases of the Composite Primary Keys As identifiers of composite entities, where each primary key combination is allowed once in the M:N relationship.As identifiers of weak entities, where the weak entity has a strong identifying relationship with the parent entity.
To illustrate the first case, assume that you have a STUDENT entity set and a CLASS entity set. In addition, assume that those two sets are related in an M:N relationship via an ENROLL entity set in which each student/class combination may appear only once in the composite entity. Shows the ERD to represent such a relationship.
The composite primary key automatically provides the benefit of ensuring that there cannot be duplicate values- that is, it ensures that the same student cannot enrol more than once in the same class.
A real-world object that is existence-dependent on another real-world object.Those types of objects are distinguishable in the real world.A dependent and an employee are two separate people who exist independently of each other.However, such objects can exist in the model only when they relate to each other in a strong identifying relationship.
A real-world object that is represented in the data model as two separate entities in a strong identifying relationship.In both situations, having a strong identifying relationship ensures that the dependent entity can exist only when it is related to the parent entity.
In summary, the selection of a composite primary key for composite and weak entity types provides benefits that enhance the integrity and consistency of the model.
Figure:5.6 The M:N relationshipbetween STUDENT and CLASS